There are many reasons I can think of why people need more than one persons advice and some are life changing. In the next few paragraphs I will explain the reasons I can think of why you need multiple peoples advice. Although I feel I need to clarify why we need advice in general. Advice is mainly helpful for learning, also it can help those who don't see what they should do, but if you got bad advice it might effect your whole life. Now that I've clarified that, time to continue with the question.

First reason, multiple viewpoints on the topic or compare and contrast. Now this reason would probably apply to someone who is seeking advice before a decision. An example of this reason : Generic_Name needs a new bike,so he goes in the bike shop and ask his friends for advice on which bike he should get and they do now. Generic_Name goes to the counter and the cashier gives him some advice so now he can compare and contrast the bikes in his head and decide which one to get. In the example Generic_Name has three point of views on the different bikes so he could easily make a choice. This is like research but from people who have done the research. Also Advice comes from friends and family mostly so you Know it isn't bad advice.

Second reason, bad advice. when you get bad advice you what to get more advice from some one else or maybe you don't even know if it is bad advice that is why always get more than one person to get advice from. Now for an example: You just turned twenty and you're still living with your parents so you go to your bad friend house because you think he has cool job probably. So you get to his house then ask him and he says you could join my gang we do cool things and get a lot a money you think about talking to your other friend but then you say yes anyway. The next day you go to jail for murder because your friend comited murder to rob a bank. You were sitting in a car waiting for your friend to get out of the bank you didn't think that he hurt or robed anyone. Now you in prison for forty years and your friend for life. When you were about to get your good friends advice you didn't but if you did you would realize how bad of an idea that is and would of been fine, so never be to careful.

Now I could list a few more but these are the main ones and most important. The first reason is useful and the second is for your safety. 